Optical filters of the interference type are well known and have been widely used in the optical and related arts. Effective as such interference optical filters may be in some instances, they nonetheless suffer from several fundamental inherent disadvantages. One such disadvantage is that interference type of optical filters inherently produce a range of attenuating effects which cannot be confined to extremely narrow bandwidths such as may be desirable in certain optical systems. That is to say that, the narrowest bandwidth of attenuation achievable by the best designed and fabricated interference optical filters may be of the order of 2 A to 5 A in width. Thus, where an optical system requires substantial attenuation of a significantly smaller bandwidth of optical energy, the interference type of optical filter is not entirely satisfactory to accomplish the desired results.
Another detracting limitation of the interference type optical filter is that it will only perform efficiently to accomplish its design objectives when it is employed in a manner to accept incident light energy within a relatively small angle of view which may be of the order of 1.degree. to 5.degree. for example. Incident light energy received by an interference type optical notch filter which is outside its narrow and limited field of view will undergo different attenuation than light energy incident within the designed field of view; moreover, the bandwidth of significantly attenuated optical energy may also be significantly and undesirably broadened for light energy incident outside the restricted relatively narrow field of view.
Additionally, the interference type optical notch filter is, by its inherent nature, a passive device. That is to say that, it cannot be turned "off" and "on" but must always operate to attenuate certain wavelengths of optical energy since it has no "on" and "off" states and operates only in a single passive mode.
More recently, advanced investigation and development work has been accomplished in the field of narrow band optical filters as may be exemplified by the work of Benjamin Senitzky disclosed in an article titled "Narrow Band Ultraviolet Vapor Filter", published January 1975, in Volume 14, No. 1 of Applied Optics beginning at page 238. The Senitzky type of filter employs a mercury vapor cell which selectively attenuates the 2537 A line over a bandwidth varying from 0.1 A to 1.0 A as a function of the internal vapor pressure of the mercury contained within the unit.
Senitzky attributes the attenuation and notch filter effects of his device to a form of selective specular reflection and suggests that the phenomena may be due to a type of re-radiation process taking place when a photon is absorbed by a ground state atom which makes a transition to an upper energy state. At a later time the atom then spontaneously emits a photon at or near the resonant wavelength and returns to its ground energy state. Thus, according to Senitzky's theorizing the operative effect of his filter is caused by a form of re-radiation taking place between the ground energy state and an upper or higher energy state of the vaporized material.
It is important to note, however, that the Senitzky device employs no electric field and is entirely a passive device which is not controllable in an "on"-"off" sense.
It is highly desirable therefore that the phenomena of extremely narrow bandwidth attenuation of certain vaporized materials be rendered available in an optical notch filter which is controllable in an on-off sense as may be required in certain optical systems. A Lidar system, for example, which is essentially an "optical radar", uses an optically responsive detector aimed at a target. The light sensitive detector must be protected from the backscatter of atmospheric light energy particularly during the period of time when a light energy pulse is initially transmitted toward a target.
Accordingly, there is a need for a conveniently controllable on-off type of optical notch filter capable of significantly attenuating extremely narrow bandwidths of light energy.